Default action is to terminate the process and dump core.
Stop
Default action is to stop the process.
First the signals described in the original POSIX.1 standard.
Signal
Value
Action
Comment
or death of controlling process
SIGINT
2
Term
Interrupt from keyboard
SIGQUIT
3
Core
Quit from keyboard
SIGILL
4
Core
Illegal Instruction
SIGABRT
6
Core
Abort signal from abort(3)
SIGFPE
8
Core
Floating point exception
SIGKILL
9
Term
Kill signal
SIGSEGV
11
Core
Invalid memory reference
SIGPIPE
13
Term
Broken pipe: write to pipe with no readers
SIGALRM
14
Term
Timer signal from alarm(2)
SIGTERM
15
Term
Termination signal
SIGUSR1
30,10,16
Term
User-defined signal 1
SIGUSR2
31,12,17
Term
User-defined signal 2
SIGCHLD
20,17,18
Ign
Child stopped or terminated
SIGCONT
19,18,25
Continue if stopped
SIGSTOP
17,19,23
Stop
Stop process
SIGTSTP
18,20,24
Stop
Stop typed at tty
SIGTTIN
21,21,26
Stop
tty input for background process
SIGTTOU
22,22,27
Stop
tty output for background process
The signals SIGKILL and SIGSTOP cannot be caught, blocked, or ignored.
Next the signals not in the POSIX.1 standard but described in SUSv2 and SUSv3 / POSIX 1003.1-2001.
Signal
Value
Action
Comment
SIGPOLL
Term
Pollable event (Sys V). Synonym of SIGIO
SIGPROF
27,27,29
Term
Profiling timer expired
SIGSYS
12,-,12
Core
Bad argument to routine (SVID)
SIGTRAP
5
Core
Trace/breakpoint trap
SIGURG
16,23,21
Ign
Urgent condition on socket (4.2 BSD)
SIGVTALRM
26,26,28
Term
Virtual alarm clock (4.2 BSD)
SIGXCPU
24,24,30
Core
CPU time limit exceeded (4.2 BSD)
SIGXFSZ
25,25,31
Core
File size limit exceeded (4.2 BSD)
Up to and including Linux 2.2, the default behaviour for SIGSYS", "SIGXCPU", "SIGXFSZ", " and (on architectures other than SPARC and MIPS) SIGBUS was to terminate the process (without a core dump). (On some other Unices the default action for SIGXCPU" and "SIGXFSZ is to terminate the process without a core dump.) Linux 2.4 conforms to the POSIX 1003.1-2001 requirements for these signals, terminating the process with a core dump.
Next various other signals.
Signal
Value
Action
Comment
SIGEMT
7,-,7
Term
SIGSTKFLT
-,16,-
Term
Stack fault on coprocessor (unused)
SIGIO
23,29,22
Term
I/O now possible (4.2 BSD)
SIGCLD
-,-,18
Ign
A synonym for SIGCHLD
SIGPWR
29,30,19
Term
Power failure (System V)
SIGINFO
29,-,-
A synonym for SIGPWR
SIGLOST
-,-,-
Term
File lock lost
SIGWINCH
28,28,20
Ign
Window resize signal (4.3 BSD, Sun)
SIGUNUSED
-,31,-
Term
Unused signal (will be SIGSYS)
(Signal 29 is SIGINFO / SIGPWR on an alpha but SIGLOST on a sparc.)
SIGEMT is not specified in POSIX 1003.1-2001, but neverthless appears on most other Unices, where its default action is typically to terminate the process with a core dump.
SIGPWR (which is not specified in POSIX 1003.1-2001) is typically ignored by default on those other Unices where it appears.
SIGIO (which is not specified in POSIX 1003.1-2001) is ignored by default on several other Unices.
Multiple instances of real-time signals can be queued. By contrast, if multiple instances of a standard signal are delivered while that signal is currently blocked, then only one instance is queued.
2.
If the signal is sent using sigqueue(2), an accompanying value (either an integer or a pointer) can be sent with the signal. If the receiving process establishes a handler for this signal using the SA_SIGACTION flag to sigaction(2) then it can obtain this data via the si_value field of the siginfo_t structure passed as the second argument to the handler. Furthermore, the si_pid and si_uid fields of this structure can be used to obtain the PID and real user ID of the process sending the signal.
3.
Real-time signals are delivered in a guaranteed order. Multiple real-time signals of the same type are delivered in the order they were sent. If different real-time signals are sent to a process, they are delivered starting with the lowest-numbered signal. (I.e., low-numbered signals have highest priority.)
If both standard and real-time signals are pending for a process, POSIX leaves it unspecified which is delivered first. Linux, like many other implementations, gives priority to standard signals in this case.
According to POSIX, an implementation should permit at least _POSIX_SIGQUEUE_MAX (32) real-time signals to be queued to a process. However, rather than placing a per-process limit, Linux imposes a system-wide limit on the number of queued real-time signals for all processes. This limit can be viewed and (with privilege) changed via the /proc/sys/kernel/rtsig-max file. A related file, /proc/sys/kernel/rtsig-nr, can be used to find out how many real-time signals are currently queued.
